This invention relates to a multi-superheterodyne receiver and, more particularly, to an improved receiver for removing residual spurious components so as to prevent reception interference.
In a receiver for receiving broadcast waves or for measuring an electric field intensity, double-superheterodyne reception is used. In this process, received waves are subjected to frequency conversion by first and second frequency converters. This process provides a high degree of wave selectivity and allows reception of weak waves without radio interference.
FIG. 1 shows a block diagram of a receiver to which such double-superheterodyne reception has been adapted. Referring to FIG. 1, reference numeral 1 denotes a high-frequency filter; 2, a first frequency converter; 3, a second frequency converter; 4, an intermediate frequency amplifier; 5, a detector; and 6, a low-frequency amplifier. First and second local oscillators 7 and 8 generate signals which are supplied to the first and second frequency converters 2 and 3. The first and second frequency converters 2 and 3 generate first and second intermediate frequencies fi1 and fi2.
When the frequency of a received wave is represented by f and the local frequency of the first local oscillator 7 is represented by fL1, the first intermediate frequency fi1 is given by: fL1-f=fi1. Similarly, when the local frequency of the second local oscillator 8 is represented by fL2, the second intermediate frequency fi2 is given by: fL2-fi1=fi2.
When two oscillators are present in a single receiver, they tend to couple electromagnetically at a common circuit portion despite shielding. This coupling generates a radio frequency component which causes radio interference.
In a receiver having a receiving frequency band of 300 to 550 MHz, when the first intermediate frequency fi1 is set at 199.75 MHz, the local frequency fL1 of the first local oscillator 7 must fall within a range of 499.75 to 749.75 MHz. At this time, when the second intermediate frequency fi2 is set at 10.7 MHz, the local frequency fL2 of the second local oscillator 8 becomes 210.45 MHz.
In this case, when the receiving frequency is 321.025 MHz, for example, the local frequency fL1 of the first local oscillator 7 becomes 321.025+199.75=520.775 MHz. When the difference between a harmonic frequency which is twice this frequency and a harmonic frequency which is four times the local frequency of the second local oscillator 8 is calculated in accordance with 1041.55-841.8=199.75 MHz, the first intermediate frequency fi1 is obtained by mutual interference between the harmonic waves and the fundamental waves of the local frequencies fL1 and fL2 of the first and second local oscillators 7 and 8. Thus, when fi1=.+-.(mfL1-nfL2) is established, residual spurious noise is generated, and beat reception or the like is caused.
In measurement of received waves, a residual spurious component may be erroneously detected as a received wave.
A description has been made with reference to an actual example of specifications in the case of residual spurious component in a double-superheterodyne receiver. However, in other frequency setttings, at least one residual spurious component is frequently generated in a single receiving band causing reception interference or erroneous reception.
This tendency is more notable in a multi-superheterodyne receiver having two or more intermediate frequencies.